Systematic constructing CoFe-Prussian blue analogue on NiCo-layered double hydroxide to obtain heterostructure two-bimetallic phosphide composite as efficient self-supported eletrocatalyst for overall water and urea electrolysis

Highlights

• Combining CoFe-PBA with NiCo-LDH on Ni foam to form hierarchical NiCoP/CoFeP@NF.

• The electrode surface exhibits superhydrophilic and superaerophobicity.

• NiCoP/CoFeP@NF shows excellent overall water and urea electrolysis performance.

Abstract

It remains a challenge to explore economical, high-effective and long term stability electrocatalysts toward large-scale hydrogen production. This work utilizes surface engineering strategy to in-situ CoFe-Prussian Blue Analogues on NiCo-layered double hydroxides to obtain 3D hierarchical heterostructure precursor (NiCo–CoFe-PBA). After phosphatization, this precursor can be further transform into tri-metallic phosphide (NiCoP/CoFeP@NF) and directly act as efficient self-supported electrode for Water and Urea Electrolysis. Impressively, the obtained NiCoP/CoFeP@NF-12 (±) electrode shows excellent catalytic performance with only requires the cell voltage of 1.61 and 1.46 V to deliver 10 mA cm⁻² in overall water splitting and urea electrolysis, respectively, which benefiting from the porous Ni foam (NF) substrate, large catalytic activity area, remarkable mass/electron transfer property, the synergistic effect of components as well as superhydrophilicity and superaerophobicity of electrode surface. In addition, the experimental results also confirm that urea-assisted system has energy saving advantage superior than traditional water splitting in alkaline electrolyte. Moreover, the hierarchical strategy can also be introduced to the construction of other intricate composites for the utilization in energy conversion and storage.

Introduction

Developing green and sustainable clean energy has received great attention because of the increasingly serious global energy crisis and environmental issues in recent years [[1], [2], [3]]. Owing to zero-carbon emission, high energy density as well as convenient storage and transportation, hydrogen energy (H₂) has been recognized as the most promising candidate for the replacement of traditional fossil fuels, and hydrogen production via water splitting (H₂O(l) → H₂(g) + 1/2 O₂(g), ΔE⁰ ≈ 1.23 V vs. RHE) has attracted considerable attention [[4], [5], [6], [7]]. Due to the slow kinetics of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), leading low energy conversion efficiency and high energy consumption in water electrolysis. Therefore, efficient catalyst is a vital challenge for achieving sustainable and economical H₂ generation. Commercial precious metal Pt-based materials have exhibited outstanding catalytic performance of HER, while IrO₂ and RuO₂ presented excellent performance for OER. Unfortunately, their scarcity, exorbitant price as well as poor durability seriously impede the industrial scale application [[8], [9], [10]]. Another concern is the electrolyte, urea electrolysis (CO(NH₂)₂(s) + H₂O(l) → N₂(g) + 3H₂(g) + CO₂(g)) to generate H₂ has captured broad attention profiting from the urea oxidation reaction (UOR) requires lower theoretical potential (0.37 V) than OER (1.23 V), resulting a low energy consumption [[11], [12], [13]]. Additionally, urea electrolysis is available for urea-rich wastewater into non-toxic N₂ and CO₂, which can be further translated to value-added products [[14], [15], [16], [17], [18]]. Accordingly, the low cost, earth abundant and highly efficient electrocatalysts have been actively explored.

Transition metal phosphides (TMP) have attracted enormous interest for their excellent electrical conductivity, fast charge transfer kinetics and good chemical stability [[19], [20], [21], [22], [23], [24], [25], [26], [27]]. For instance, McEnaney et al. demonstrated that amorphous MoP nanoparticles exhibited superior HER performance than bulk MoP [19]. Wang et al. synthesized porous Ni₂P as bifunctional electrocatalysts, which has shown excellent electrocatalytic activity for both HER and OER in alkaline medium [20]. Driess et al. reported amorphous CoP with good overall water splitting performance [25]. Nevertheless, the powdery-type electrocatalysts still suffer from localized limitations, such as low loading mass, prone to agglomerate and inevitably use polymer binder and so on [28,29]. Growing catalytically active materials on conductive substrates to obtain self-supported catalysts has been regarded as an effective strategy to solve these problems [27]. Recent reports have shown the great potential of layered double hydroxides (LDHs) materials in the field of water electrolysis, especially NiCo-LDH [[30], [31], [32], [33], [34], [35], [36]]. However, it cannot be ignored that there are problems of poor conductivity and sparse catalytic active sites on the edge, which leads to reduced catalytic activity [34]. While growing the ordered and continuous LDHs-based materials on conductive substrate to obtain self-supported electrocatalysts can not only improve the conductivity and facilitate the mass and charge transportation, but also accelerate the separation of gas bubbles [29,37,38].

Additionally, utilizing surface engineering strategy to construct hierarchical structure can significantly improve the catalytic performance, which can be ascribed to the following advantages: (1) increase the active surface area and edge catalytic active sites; (2) strong synergistic effect; (3) adjust electronic structure and reduce adsorption energy [30,39]. Specifically, hierarchical structured electrocatalysts (MnCo₂O_4.5@Ni(OH)₂ [17], Co-Z/Se-2 [18], NiFe LDH/Cu(OH)₂/Cu [33], NC-PB@CNT [40], Co@N-CS/N-HCP@CC [41]), which provide more active sites, higher surface area, faster electronic transmission capability as well as better electrochemical performance compared with their single structure. As the secondary structure, Prussian blue analogues (PBAs) have been considered as promising candidates in electrocatalytic water splitting, benefiting from the convenient fabrication, controllable compositions (single-metallic, bimetallic), homogeneous catalytic active sites as well as readily preparation of transition metal phosphides, selenides, alloy, etc [40,[42], [43], [44], [45], [46], [47]].

Inspired by the above mentioned, we rationally combined the most commonly used CoFe-PBA cubes with ultrathin NiCo-LDH to form 3D hierarchical heterostructure NiCoP/CoFeP@NF, as a previously unreported self-supported bifunctional electrocatalysts. For the fabrication of NiCoP/CoFeP@NF electrodes, NiCo-LDH nanosheets array firstly grown on NF surface through hydrothermal method, and then the CoFe-PBA nanocubes further in-situ grown on the surface of NiCo-LDH through facile ageing method to form the NiCo–CoFe-PBA precursors. Finally, these precursors were phosphatized to obtain self-supported NiCoP/CoFeP@NF electrodes. Desirably, the as-synthesized NiCoP/CoFeP@NF electrodes exhibited outstanding catalytic activities, especially NiCoP/CoFeP@NF-12 (±) has the best catalytic performance that only require as low cell voltages as 1.61 and 1.45 V to deliver 10 mA cm⁻² for overall water splitting and urea electrolysis, with remarkable durability for 40 h, respectively. Therefore, the heterostructure NiCoP/CoFeP@NF-12 electrode holds great promising to apply as alternative precious metals for industrial-scale overall water and Urea Electrolysis, and this surface engineering strategy is also universal for the fabrication of other hierarchical heterostructural composites.